JP7284194B2 - Metal oxide-based electrode composition - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This invention is the subject of GB 1809467.2 filed on June 8, 2018 (08.06.2018) and dated April 12, 2019 ( GB 1905218.2 filed on 12.04.2019) and claims these advantages.

The present invention provides electrodes and electrochemical cells, such as lithium ion batteries, containing the electrodes, and methods of using the electrodes in electrochemical cells.

Fast lithium-ion battery electrode materials are capable of storing large amounts of charge in minutes rather than hours of charging. Such materials are needed to solve the technical challenges associated with the adoption of electric vehicles and grid-scale batteries and to enable new power enhancement devices.

The most intuitive and commonly used approach to improving the rate performance of electrode materials is to create nano-sized or porous (and often hierarchical) structures. This allows for more rapid intra-electrode transport of lithium ions by minimizing the solid-state diffusion length of lithium ions, increasing the surface area of the electrode material in contact with the electrolyte. Carbonaceous hierarchical structures and carbon coatings are also frequently used to improve electrical conductivity, another essential requirement for high speed applications.

In fact, graphite, despite its excellent lithium ion mobility, cannot be used at high speeds due to the risk of particle breakage and the formation of lithium dendrites resulting in short circuits and ignition and explosion ( Zhao et al.; Downie et al.). The latter issue inherently limits the use of low voltage anodes in high speed applications, as electrode non-uniformity, or any source of increased overpotential, can lead to lithium plating potentials on the surface of the low voltage electrode. (Downie et al.).

Li ₄ Ti ₅ O ₁₂ (lithium titanate; LTO), which has an average voltage of 1.55 V versus Li ⁺ /Li, is associated with an undesirable but necessary reduction in full-cell voltage and thus energy density. It allows rapid (de)intercalation without the risk of lithium dendrites or substantial solid-electrolyte interface (SEI) formation, if at all. However, even in this well-established “fast” anode, the capacity of 1 μm particles from solid-state synthesis reaches only 60-65 mA h g ⁻¹ at 10 C rate (Kim et al. ).

Several strategies have been proposed to increase the capacity of LTO electrodes at high C-rates, and carbon-coated nanoparticles of current LTO can reach at least 150 mA h g ⁻¹ at 10 C ( Wang et al.; Odziomek et al.). This corresponds to approximately 0.5 lithium ions per transition metal (Li ⁺ /TM).

However, the use of nanostructured and porous materials for electrochemical energy storage applications inherently poses significant penalties in terms of volumetric energy density. Moreover, these carefully engineered porous and nanoarchitectures are time consuming and expensive to synthesize, characterize and manufacture. Synthetic methods often result in relatively low yields and/or the generation of large amounts of chemical waste (Oszajca et al.). Furthermore, these porous and nanoarchitectures are also associated with catalytic decomposition of electrolytes (Palacin et al.), morphological changes leading to loss of nanostructuring (Wu et al.), and higher first cycle capacity losses (Kasnatscheew et al.), etc. from the process, it is susceptible to degradation during electrochemical cycling.

Another problem with the use of nano-LTO electrodes in full cells is gassing and associated swelling or pressure build-up during repeated charge and discharge cycles. This results from heterogeneous catalysis between the metal oxide surface and the organic electrolyte (He et al.; Lv et al.). The small particle size needed to compensate for the poor lithium ion and electron diffusion in LTO increases the reactive surface area and exacerbates this problem.

Rapid charging or high power delivery from a full cell also requires matching the cathode to the anode. _LiFePO4 has been used as a promising high speed cathode (Zaghib et al.). However, both _LiFePO4 and LTO have exceptionally flat voltage profiles. Although this combination provides constant voltage, it poses a significant challenge with respect to battery management systems (BMS). A simple and accurate BMS is a decisive factor for battery applications in electric vehicles and mobile technology, at high speeds to prevent dangerous and degradative overcharging (discharging) while maximizing utility. is even more important to BMS relies on the ability to measure the state of charge, which cannot be easily done by charge measurement alone, as the battery degrades and electron-consuming side reactions occur.

In view of the above challenges, there is a need to provide new electrode materials for lithium-ion batteries that are capable of operating at high speeds.

The present invention relates generally to electrodes comprising niobium tungsten oxide, electrochemical cells comprising such electrodes, and e.g. A use of the cell is provided.

The inventors have demonstrated that extremely high volumetric energy densities and excellent charge and discharge rates can be achieved using electrode materials containing niobium tungsten oxide in high speed lithium ion batteries. Niobium tungsten oxide has favorable lithium diffusion properties and thus exhibits superior performance. Above 1.0 V vs. Li ⁺ /Li, SEI formation is minimal, meaning that lithium is not lost to side reactions with the electrolyte.

In addition, high speed lithium ion batteries using electrode materials containing niobium tungsten oxide can operate above 1.3V. (For example, the average voltage of Nb ₁₆ W ₅ O ₅₅ is 1.57 V vs. Li ⁺ /Li and the average voltage of Nb ₁₈ W ₁₆ O ₉₃ is 1.67 V vs. Li ⁺ /Li). Operating in this voltage range negates the need to perform initial formation cycles and simplifies the battery manufacturing process. A typical lithium-ion battery containing graphite electrodes operates below 1.3 V and must undergo an initial formation cycle before the battery can be sealed. Typically, this forming cycle is performed at an elevated temperature, such as 60° C., to cause outgassing. This adds considerable time and cost to the battery manufacturing process.

Furthermore, in full cells, for example, _LiFePO4 , LiN( _CF3SO2 ) ₂ ( _LiTFSI ) is used instead of the more toxic _LiPF6 electrolyte salt normally used in standard commercial electrolytes. be able to. Furthermore, aluminum can be used as a current collector instead of the more expensive copper while avoiding the LiAl alloy potential (≤0.3 V vs. Li ⁺ /Li).

The use of niobium tungsten oxide as an electrode material allows open circuit voltage to be used as a measure of state of charge, for example in high speed lithium batteries. This has the potential to provide a simple and reliable BMS and may prove to be a great advantage for high power/fast charging applications.

In a first aspect of the invention, a method of charging and/or discharging an electrochemical cell at a C-rate of at least 5C, such as at least 10C, the electrochemical cell having a working electrode comprising niobium tungsten oxide, A method is provided.

An electrochemical cell may include a counter-electrode and an electrolyte, optionally the electrodes are connectable or connected to a power source.

The method may be a method of charging and/or discharging the electrochemical cell at a current density of at least 750 mA·g ⁻¹ , such as at least 800 mA·g ⁻¹ .

The method may comprise cycling the electrochemical cell to charge and discharge or discharge and charge, the method comprising 2 cycles or more, 5 cycles or more, 10 cycles or more, 50 cycles or more, 100 cycles or more, 500 cycles or more. , 1000 cycles or more, or 2000 cycles or more.

In one embodiment, the working electrode does not have a porous structure or a hierarchical structure.

The working electrode may comprise niobium tungsten oxide in particulate form. For example, niobium tungsten oxide may have a primary grain size of at least 1 μm.

The smaller surface area of particulate niobium tungsten oxide reduces side reactions observed with nano-LTO electrodes and alleviates gassing and associated swelling or pressure build-up problems. Furthermore, particulate niobium tungsten oxide can be prepared quickly and easily using solid state synthesis.

The working electrode may have a solid-state lithium diffusion coefficient (D _Li ) of less than 10 ⁻¹⁴ m ² ·s ⁻¹ at 298K, such as less than 10 ⁻¹⁵ m ² ·s ⁻¹ at 298K.

Favorable lithium diffusion properties allow micrometer-sized particles of niobium tungsten oxide to be used at extremely high speeds.

The working electrode may have a capacity of 50 mA·h·g ⁻¹ at 20C. For example, 75 mA·h·g ⁻¹ at 20C or 50 mA·h·g ⁻¹ at 60C.

The molar ratio of _Nb2O5 and _WO3 in the working electrode may be from 6:1 to 1:15 _. For example, from 8:5 to 11:20.

_{The working electrode was Nb12WO33 , Nb26W4O77 , Nb14W3O44 , Nb16W5O55 , Nb18W8O69 , Nb2WO8 , Nb18W16O93 , Nb Niobium tungsten} selected _{from the group consisting of 22W20O115 , Nb8W9O47 , Nb54W82O381 , Nb20W31O143 , Nb4W7O31} , _{or Nb2W15O50} It may contain an oxide. _{For example , Nb16W5O55 , Nb18W8O69 , Nb2WO8 , Nb18W16O93 , or Nb22W20O115 .}

Additionally, the working electrode may comprise a mixture of niobium tungsten oxide and an additional active material. For example, the working electrode may comprise a mixture of niobium tungsten oxide and LTO. The ratio of niobium tungsten oxide to LTO may be from 95:5 to 5:95 by weight. For example, the ratios are 90:10 to 10:90 by weight, 80:20 to 20:80 by weight, 70:30 to 30:70 by weight, 60:40 to 40:60 by weight. Alternatively, the ratio of niobium tungsten oxide to LTO may be 1:1 by weight.

In a further aspect of the invention, an electrode, also called working electrode, comprising niobium tungsten oxide, wherein for example the molar ratio of Nb ₂ O ₅ and WO ₃ in the electrode is from 8:5 to 11:20 , electrodes are provided. The working electrode is suitable for use in lithium ion batteries.

The working electrode may comprise _{a niobium tungsten oxide} selected _{from Nb16W5O55 , Nb18W8O69 , Nb2WO8 , Nb18W16O93} , or _{Nb22W20O115 .} For example Nb ₁₆ W ₅ O ₅₅ or Nb ₁₈ W ₁₆ O ₉₃ .

In one embodiment, the working electrode does not have a porous structure or a hierarchical structure.

The working electrode may comprise niobium tungsten oxide in particulate form. For example, niobium tungsten oxide may have a primary grain size of at least 1 μm.

Additionally, the working electrode may comprise a mixture of niobium tungsten oxide and an additional active material. For example, the working electrode may comprise a mixture of niobium tungsten oxide and LTO. The ratio of niobium tungsten oxide to LTO may be from 95:5 to 5:95 by weight. For example, the ratios are 90:10 to 10:90 by weight, 80:20 to 20:80 by weight, 70:30 to 30:70 by weight, 60:40 to 40:60 by weight. Alternatively, the ratio of niobium tungsten oxide to LTO may be 1:1 by weight.

In a further aspect of the invention there is provided an electrochemical cell comprising the working electrode of the invention.

In a further aspect of the invention there is provided a lithium ion battery comprising one or more electrochemical cells of the invention. When multiple cells are present, the cells may be provided in series or parallel.

In a further aspect of the invention there is provided the use of a working electrode comprising niobium tungsten oxide in a fast electrochemical cell, for example where the cell is operated at a C-rate of at least 5C, such as at least 10C.

In a further aspect of the invention, a method of charging and/or discharging an electrochemical cell at a C-rate of at least 5C, such as at least 10C, wherein the electrochemical cell has a working electrode comprising niobium molybdenum oxide. is provided.

The inventors have found that high energy densities and excellent speeds can be achieved using electrode materials comprising niobium molybdenum oxide in high speed lithium ion batteries. Niobium molybdenum oxide exhibits a higher average voltage than typical high speed anode materials, thus avoiding or minimizing interaction with the electrolyte.

In a further aspect of the invention, an electrode, also referred to as _working electrode, comprising niobium molybdenum oxide, wherein, for example, the molar ratio of _Nb2O5 and _MoO3 in the electrode is from 6:1 to 1:3, Electrodes are provided, for example 1:3. The working electrode is suitable for use in lithium ion batteries.

The _working electrode may comprise a _{niobium molybdenum} oxide selected _{from Nb2Mo3O14} , _Nb14Mo3O44 , or _{Nb12MoO44 . For} example, _{Nb2Mo3O14 .}

These and other aspects and embodiments of the invention are described in greater detail below.

FIG. 1 is a low magnification wide-field SEM image of (A) Nb ₁₆ W ₅ O ₅₅ and (B) Nb ₁₈ W ₁₆ O ₉₃ , showing a nearly uniform grain size and the presence of agglomerates. The background of (A) is Cu grid and the background of (B) is adhesive carbon. Scale bar is 50 μm in both images. FIG. 2 is the atomic structure and SEM images showing (AC) Nb ₁₆ W ₅ O ₅₅ and (DF) Nb ₁₈ W ₁₆ O ₉₃ . Scale bars shown in (B) and (E) are 5 μm. Scale bars shown in (C) and (F) are 10 μm. FIG. 3 is a diagram showing the bronze-like structure of Nb ₁₈ W ₁₆ O ₉₃ and its relation to classical tetragonal tungsten bronze (TTB). Partially filled spheres represent K ⁺ ions that are not present in Nb ₁₈ W ₁₆ O ₉₃ . FIG. 4 is a diagram showing the rate performance of bulk Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ . (A) Galvanostatic discharge and charge of Nb ₁₆ W ₅ O ₅₅ from 60 C (leftmost curve at 1.0 V and 3.0 V) to 0.2 C (rightmost curve at 1.0 V and 3.0 V) Curves; (B) dQ/dV of Nb ₁₆ W ₅ O ₅₅ from 0.2C (leftmost curve at 50mA·h·g ⁻¹ ) to 60C (rightmost curve at 50mA·h·g ⁻¹ ). Plot; (C) Galvanostatic discharge of _Nb18W16O93 from _100C (leftmost curve _at 1.0V and 3.0V) to 0.2C (rightmost curve at 1.0V and 3.0V) and charging curves; (D) dQ of Nb ₁₈ W ₁₆ O ₉₃ from 0.2 C (leftmost curve at 50 mA·h·g ⁻¹ ) to 100 C (rightmost curve at 50 mA·h·g ⁻¹ ). /dV plot; (E) Summary of rate performance and (F) 250 cycles at 10C followed by 750 cycles of fast cycling at 20C. FIG. ₅ shows _the voltage profiles _{of Li4Ti5O12} (curve with plateau at 1.55 _V ) _{, Nb16W5O55} (curve crossing at 1.57V) and _{Nb18W16O93 .} Voltage profiles (curves that intersect at 1.65 V) are shown. Discharge and charge voltage profiles are normalized to (A) gravimetric capacity and (B) lithium ions transferred per transition metal atom. The average voltages _{of Li4Ti5O12} , _Nb16W5O55 and _Nb18W16O93 are _1.55V , _1.57V and 1.65V _{, respectively .} The Li ₄ Ti ₅ O ₁₂ electrochemistry shown here (extracted by WebPlotDigitizer 2017, Ankit Rohatgi, Inc.) shows optimal performance, 15-20 nm carbon nanofibers (CNF) at a mass loading of 1 mg cm ⁻² . It consists of 5-20 nm particles on top. FIG. 6 is a comparison of Cu foil and carbon-coated Al foil current collectors. FIG. 6A shows Nb ₁₆ W ₅ O ₅₅ subjected to 1000 cycles under galvanostatic discharge and charge at 10 C (first 200 cycles) and 20 C (remaining 800 cycles); Cu foil at 1.2 V (second from top); C--Al at 1.0 V (third from top) and C--Al at 1.2 V (bottom line). FIG. 6B shows discharge and charge profiles between 3.0 and 1.0 V for the 100th cycle at 10 C; curve order at 100 mA·h·g ⁻¹ : C—Al charge curve (top); Cu foil charge curve (second from top); Cu foil discharge curve (third from top) and C—Al discharge curve (bottom). FIG. 7 shows longer term cycling as a function of minimum cutoff voltage. (A) Nb ₁₆ W ₅ O ₅₅ cycled at C/5 (first 50 cycles) and 1 C (second 50 cycles) on Cu foil with 1 hour potentiostatic hold at top of charge. V _min =1.2 V (top curve); V _min =1.0 V (bottom curve). (B) Nb ₁₆ W ₅ O ₅₅ cycled at C/5 on Al foil without potentiostatic charging step. FIG. 8 shows the overpotential in Li||Li symmetric cells as a function of current density. FIG. 9 shows the stimulated echo pulse magnetic field gradient sequence used to measure the diffusivity of ⁷ Li exhibiting both radio frequency ( ⁷ Li) and magnetic field gradient (G _z ) pulses. Here, the gradient pulse duration (t _g ) includes the ramp-up, on-time, and ramp-down of the opt composite gradient pulse. FIG. 10 shows (A) Li _10.2 Nb ₁₈ W ₁₆ O ₉₃ at 453 K, (B) Li _6.8 Nb ₁₈ W ₁₆ O ₉₃ at 453 K, and (C) Li _3.4 Nb ₁₈ W at 453 K. _16O93 , (D) _{Li6.3Nb16W5O55 at 353 K ,} and (E) _{Li8.4Nb16W5O55} at ₃₈₃ K as a function of gradient _{strength .} A representative ⁷ Li decay curve showing NMR signal intensity is shown. Black circles indicate experimental data points and red lines indicate (AC; E) monoexponential or (D) biexponential fit of the data to the Stejskal-Tanner equation. FIG. 11 shows lithium diffusion measured by ⁷ Li pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The lithium diffusion coefficients of Li _6.3 Nb ₁₆ W ₅ O ₅₅ and Li _x Nb ₁₈ W ₁₆ O ₉₃ (x=3.4, 6.8, 10.2) were measured in the temperature range 333-453K. Filled ( ₈₅ % signal contribution) and _unfilled (15% signal contribution ₎ symbols for _{Li6.3Nb16W5O55} correspond to the observed binary diffusion. FIG. 12 shows: (A) relative change in lithium diffusion as a function of open circuit voltage (Voc) and (B) full discharge was reached in about 1 month with a 12 hour rest period at each point. Open circuit voltage versus closed circuit voltage (Vcc) from galvanostatic intermittent titration (GITT) measurements showing the "thermodynamic" electrochemical profile at C/20. FIG. 13 shows (A) Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ from this study compared to high speed electrode formulations from the literature. Volumetric capacities at 1C and 20C are determined from reported capacities and reported, measured or estimated tap densities. (B) Ragone (log-log) plot of energy density and power density based on anode active material vs. 4.0V cathode. The mass loading of niobium tungsten oxide was here 2.6 mg·cm ⁻² and the mass loading of other titanium and niobium based materials was typically about 1 mg·cm ⁻² . Graphite cannot be used in high speed applications due to the risk of lithium plating and particle breakage, but is included as a reference. Literature values for Ragone plots were extracted using WebPlotDigitizer. FIG. 14 shows the theoretical volumetric capacity assuming a reaction and crystallographic (single crystal) density of one electron per transition metal. The capacity of _Li4Ti5O12 is shown for the well-established reaction to _Li7Ti5O12 ; the _{capacity of} graphite is based on _the reaction of _one electron per _C6 unit. FIG. 15 shows the capacity versus cycle number of Nb ₂ Mo ₃ O ₁₄ at 0.05 C and 1.0 to 3.0 V vs. Li ⁺ /Li. Figure 16 shows the discharge and charge voltage profiles of _{Nb2Mo3O 14} within three different voltage windows (1.0V, 1.2V and 1.4V) _. Discharge and charge curves are normalized to gravimetric capacity; curve order at 200 mA·h·g ⁻¹ : 1.4 V charge curve (top); 1.2 V charge curve (second from top); 1.0V charge curve (third from top); 1.2V discharge curve (third from bottom); 1.0V and 1.4V discharge curves (partially overlapping; bottom). _FIG . 17 shows the discharge _and charge voltages _of (A) NMC622 ( _{LiNi0.6Co0.2Mn0.2O2} ), (B) _LiFePO4 , and ( _C ) _LiMn2O4 versus Li/Li ⁺ . Show your profile. FIG. 18 compares the capacity at various charge and discharge rates for electrodes containing Nb ₁₆ W ₅ O ₅₅ and various separators, binders, binder amounts, conductive carbon types, and electrode calendering. FIG. 19 shows the discharge and charge voltage profiles of a full cell containing a Nb ₁₆ W ₅ O ₅₅ anode and (A) LiFePO ₄ , (B) LiMn ₂ O ₄ and (C) NMC622 cathodes. Figure 19 (D-E) shows the rate performance of a cell containing a Nb ₁₆ W ₅ O ₅₅ anode and an NMC622 cathode; the order of the curves in (E): capacity (top, sloped); coulombic efficiency ( downward, horizontal). FIG. 20 shows (A) long-term cycling at 1C charge and discharge rates of a cell containing a Nb ₁₆ W ₅ O ₅₅ anode and NMC622 cathode; capacity (tilted); coulombic efficiency (horizontal). (B) shows the performance of the extracted NMC622 electrode on Li/Li ⁺ . (C) shows the performance of the extracted Nb ₁₆ W ₅ O ₅₅ electrode on Li/Li ⁺ . FIG. 21 shows (A) charge and discharge profiles of Nb ₁₆ W ₅ O ₅₅ -LTO mixtures at ratios of 7:3, 1:1, and 3:7, normalized to gravimetric capacity; Order of charge curves at 0V: 3:7 (leftmost), 5:5 (second from left), 7:3 (rightmost); Order of discharge curves at 1.0V: 3:7 (leftmost), 5:5 (second from left) and 7:3 (far right). (B) Rate performance of mixtures, in order at 0.2C: 7:3 (top), 5:5 (second from top), 3:7 (bottom). (C) Longer cycle performance at 10C charge and discharge rates, 1C points: 7:3 (top), 5:5 (second from top), 3:7 (bottom); 10C curves: 3 : 7 (top), 5: 5 (second from top), 7: 3 (bottom). FIG. 22 shows (A) the charge and discharge profile of a full cell containing a 3:7 (Nb ₁₆ W ₅ O ₅₅ :LTO) anode and NMC622 cathode and (B) the rate performance of the cell.

The present invention relates generally to electrodes comprising niobium tungsten oxide, electrochemical cells comprising such electrodes, and the use of such cells at high C-rates of 5 C during charge and/or discharge, for example in lithium ion batteries. offer.

The preparation of Nb ₁₆ W ₅ O ₅₅ has been previously described, inter alia, by Roth and Wadsley. The preparation of Nb ₁₈ W ₁₆ O ₉₃ has been previously described, inter alia, by Stephensen. However, the electrochemical properties of Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ are not described in these documents.

_Nb8W9O47 ( _Montemayor et al _{. )} ; _{Nb26W4O77 (} Fuentes _et al _. ); Nb14W3O44 (Fuentes _et al _{. )} ; _-x O _3-x/2 (Yamada et al.), has been previously described. However, the material's capacity to C-rate was not measured.

Electrodes comprising Nb _8-x W _9-x O ₄₇ where 1≦x≦6 have been described by Cruz et al. However, electrochemical cells containing such electrodes have been used under limited conditions and there is no disclosure of cell operation under high speed conditions. Furthermore, the authors report that irreversible structural transformations in matrix-host lead to capacity loss after the first cycle.

Electrodes comprising Nb ₁₂ WO ₃₃ have been described by Saritha et al. and Yan et al. Yan et al. tested an electrochemical cell containing electrospun Nb ₁₂ WO ₃₃ electrodes at a maximum current density of 700 mA·g ⁻¹ (corresponding to a C-rate of 3.6 C). Saritha et al. have tested electrochemical cells containing Nb ₁₂ WO ₃₃ at reported C-rates of 20C or less.

However, Saritha et al. clearly define the C-rate as the reaction (ie, desorption or insertion) of one lithium ion in one hour. This corresponds to one electron transfer per chemical formula unit. Therefore, the 20C rate for Nb ₁₂ WO ₃₃ reported by Saritha et ^al .

The inventors have developed an electrode comprising niobium tungsten oxide that has favorable lithium ion diffusion properties and thus provides excellent performance even with micron-sized particles. The electrodes exhibit extremely high volumetric energy densities and high capacities at fast charge and discharge rates.

Voltage values described herein are made relative to Li ⁺ /Li, as is common in the art.

C-rate is a measure of the rate at which a battery is discharged to its maximum capacity. C-rate can be defined as the reciprocal of the number of hours to reach a specified theoretical capacity, eg, 10C corresponds to 6 minutes of discharge or charge time. In this work, ^the C-rate is defined for _one electron transfer per _transition metal ^, _e.g. ¹ . Theoretical capacity is calculated by:

where n is the number of electron transfers per chemical formula unit, F is the Faraday constant, 3.6 is the conversion factor between coulombs and normal mA·h·g ⁻¹ , and m is the chemical formula Mass per unit. Therefore, a 1C rate corresponds to the reaction (i.e., insertion or removal) of 21 lithium ions per chemical formula unit of Nb ₁₆ W ₅ O ₅₅ per hour, which means that this material can produce 21 lithium ions per chemical formula unit. This is because it contains a transition metal.

High speed applications may also be described, for example, by relating to (weight) current densities where the current density is at least 800 mA·g ⁻¹ or 1000 mA·g ⁻¹ . Current density is related to C-rate by:

Thus, using the conventions defined in this study, a current density of 800 mA·g ⁻¹ corresponds to a C-rate of 4.67 C for Nb ₁₆ W ₅ O ₅₅ and 800 mA for Nb ₁₈ W ₁₆ O ₉₃ . • A current density of g ⁻¹ corresponds to a C-rate of 5.36C.

All (weight) capacities are quoted based on the mass of the active electrode material.

Working Electrode The present invention provides a working electrode comprising niobium tungsten oxide. The working electrode is electrically conductive and can be electrically connected to a counter electrode, eg, in an electrochemical cell.

The working electrode may be the anode or the cathode during the discharge step, for example in a lithium ion battery. Typically the working electrode is the anode during the discharge step.

Typically, the working electrode used in the method contains a molar ratio of Nb ₂ O ₅ and WO ₃ from 6:1 to 1:15. Preferably, the molar ratio _{of Nb2O5} and _WO3 in the working electrode is from 8:5 to 11:20.

Typically, _the working electrode used _{in the method is Nb12WO33} , _{Nb26W4O77 , Nb14W3O44 , Nb16W5O55 , Nb18W8O69 , Nb2WO 8 , Nb18W16O93 , Nb22W20O115 , Nb8W9O47 , Nb54W82O381 , Nb20W31O143 , Nb4W7O31 , or Nb2W15O _ _ _ _ _ 50} , or a niobium tungsten oxide selected from combinations thereof. _Preferably , _{the working} electrode _{comprises Nb16W5O55} , _{Nb18W8O69 , Nb2WO8 , Nb18W16O93} , or _Nb22W20O115 , _or combinations _{thereof .}

Typically, the molar ratio of Nb ₂ O ₅ and WO ₃ in the working electrode is 8:5 to 11:20. Preferably, _the molar ratio of _Nb2O5 and _WO3 in the working electrode is 8:5 or 9:16.

_Typically , _{the working} electrode comprises _Nb16W5O55 , _{Nb18W8O69 , Nb2WO8 , Nb18W16O93 ,} or _{Nb22W20O115 ,} or combinations _{thereof .} Preferably, the working electrode _{comprises Nb16W5O55} or _Nb18W16O93 , or _a combination _{thereof .}

Optionally, the working electrode comprises a mixture of niobium tungsten oxide and additional active material. The additional active material may be an additional metal oxide. For example, the working _electrode can be niobium tungsten oxide, lithium titanate ( _{LTO; Li4Ti5O12), titanium niobium oxide (e.g. TiNb2O7 ) ,} titanium tantalum oxide _{( e.g. TiTa2O7} ). , tantalum molybdenum oxide (eg Ta ₈ W ₉ O ₄₇ ), and niobium molybdenum oxide (eg Nb ₂ Mo ₃ O ₁₄ ).

Graphite may also be used as an additional active material. A working electrode comprising a mixture of niobium tungsten oxide and graphite is less expensive to manufacture while maintaining the beneficial properties outlined above.

Preferably, the working electrode comprises a mixture of niobium tungsten oxide and LTO. The ratio of niobium tungsten oxide to LTO may be from 95:5 to 5:95 by weight. For example, the ratios are 90:10 to 10:90 by weight, 80:20 to 20:80 by weight, 70:30 to 30:70 by weight, 60:40 to 40:60 by weight. Alternatively, the ratio of niobium tungsten oxide to LTO may be 1:1 by weight.

Preferably, the working electrode consists essentially of niobium tungsten oxide and an additional active material. For example, the working electrode consists essentially of a mixture of niobium tungsten oxide and LTO.

Typically, the working electrode does not have a porous or hierarchical structure. For example, the electrode material may be less than 20 m ² ·g ⁻¹ , less than 10 m ² ·g ⁻¹ , less than 5 m ² ·g ⁻¹ , less than 3 m ² ·g ⁻¹ , less than 2 m ² ·g ⁻¹ , or 1 m 2 ·g ⁻¹ It may have a specific surface area of less than g ⁻¹ .

The specific surface area of the electrode material may be known or determined using standard techniques such as N ₂ adsorption isotherm analysis and Brunauer-Emmett-Teller (BET) theory.

Alternatively, the working electrode may have a porous structure. For example, the working electrode may be at least 50 m ² ·g ⁻¹ , at least 60 m ² ·g ⁻¹ , 70 m ² ·g ⁻¹ , 80 m ² ·g ⁻¹ , 90 m ² ·g ⁻¹ , 100 m ² ·g ⁻¹ , It may have a specific surface area of 150 m ² ·g ⁻¹ , 200 m ² ·g ⁻¹ , 300 m ² ·g ⁻¹ , or 400 m ² ·g ⁻¹ .

The working electrode is at least 0.1 cm ³ ·g ⁻¹ , at least 0.2 cm ³ ·g ⁻¹ , at least 0.4 cm ³ ·g ⁻¹ , at least 0.5 cm ³ ·g ⁻¹ , at least 0.7 cm ^{3 ·} g −1 . g ⁻¹ , at least 0.8 cm ³ ·g ⁻¹ , at least 0.9 cm ³ ·g ⁻¹ , at least 1.0 cm ³ ·g ⁻¹ , at least 1.5 cm ³ ·g ⁻¹ , or at least 2.0 cm ³ • May have a pore volume of g ⁻¹ .

The pore volume of the electrode material may be known or determined using standard techniques such as N ₂ adsorption isotherm analysis and Barrett-Joyner-Halenda (BJH) theory.

The porous working electrode may have an average pore size (largest cross section) of at least 1 nm, at least 5 nm, at least 10 nm, at least 20 nm, at least 30 nm, at least 40 nm, at least 50 nm, or at least 100 nm.

The porous working electrode may have a macroporous structure. Accordingly, a porous working electrode may comprise pores having a maximum cross-section of at least 200 nm, at least 500 nm, at least 1 μm, or at least 5 μm.

The pore size of the electrode material may be known or determined using standard techniques such as scanning electron microscopy (SEM).

The working electrode may further comprise porous carbon, such as porous reduced graphene oxide.

Electrodes comprising porous carbon are generally lightweight and conductive, and can provide larger pore volumes, which can allow rapid transport of lithium ions and electrons into the active material. . They may also increase the electrochemical capacity of the working device.

The working electrode may further comprise reduced graphene oxide, Ketjenblack or Super P carbon.

Alternatively, the working electrode may have a hierarchical structure. For example, the working electrode may further comprise hierarchically reduced graphene oxide (rGO).

Typically, the working electrode comprises niobium tungsten oxide in particulate form. The size of the niobium tungsten oxide particles of the working electrode may be known or determined using standard techniques such as SEM.

Typically, the niobium tungsten oxide particles of the working electrode have a primary particle size of at least 1 μm. Primary particle size is the size of an individual crystallite. Primary particle size is the smallest recognizable subdivision in a particle system. For example, niobium tungsten oxide particles have a primary particle size of at least 2 μm, 3 μm, 4 μm, 5 μm, or 10 μm.

Individual niobium tungsten oxide particles may agglomerate to form secondary particles. Typically the niobium tungsten oxide particles have an agglomerate (secondary) particle size of at least 5 μm. More preferably, the niobium tungsten oxide has an agglomerate particle size of at least 10 μm, 15 μm, 20 μm, 25 μm, or 30 μm.

Additional active material, if present, may be in particulate form. The size of the additional active material particles may be known or determined using standard techniques such as SEM.

Preferably, the additional active material particles have a primary particle size of 1 μm or less. For example, the additional active material particles have a primary particle size of 800 nm or less, 750 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, 200 nm or less, or 150 nm or less. Particulate lithium titanate typically has a particle size within this range.

Electrodes containing mixtures of niobium tungsten oxide and additional active materials having particle sizes within the above ranges can be charged and discharged with very high C-rates and very high charge densities.

Conductive carbon materials (eg, carbon black, graphite, nanoparticulate carbon powder, carbon fibers and/or carbon nanotubes) are typically mixed with the working electrode material to improve conductivity in the working electrode. Alternatively, a conductive carbon material may be coated onto the working electrode material. In one embodiment, the working electrode comprises porous carbon, eg, porous reduced graphene oxide, which may cover larger niobium oxide particles.

Typically, the working electrode contains 1-5% by weight binder.

The electrodes consist essentially of niobium tungsten oxide.

Alternatively, the working electrode is mixed with a binder or adhesive. Some examples of binders or adhesives include PVDF, PTFE, CMC, PAA, PMMA, PEO, SBR, and copolymers thereof.

The working electrode is typically fixed to a current collector, for example a copper or aluminum collector, which may be in the form of a plate.

The inventors found that in a 2032 type coin cell configuration with an active material loading of 2-3 mg·cm ² and an electrode area of 1.27 cm ² for a lithium counter electrode, an active material/carbon/binder ratio of 8: A working electrode comprising particulate niobium tungsten oxide was evaluated using a standard electrode configuration of 1:1 and using 1.0 M LiPF ₆ in ethylene carbonate/dimethyl carbonate as the electrolyte.

Under these conditions, we found that working electrodes containing niobium tungsten oxide produced up to 150 mA·h·g ⁻¹ at 10 C for 1000 cycles and up to 125 mA·h·g −1 at 20 C for 750 cycles ^{. 1} can be maintained.

Additionally, the inventors have found that a working electrode comprising niobium tungsten oxide has a voltage profile that is sloped rather than flat.

We have evaluated solid-state lithium diffusion in niobium tungsten oxide using both pulsed field gradient NMR (PFG NMR) and galvanostatic intermittent titration (GITT). The inventors have found that niobium tungsten oxide has a solid-state lithium diffusion coefficient (D _Li ) of 10 ⁻¹³ to 10 ⁻¹² m ² ·s ⁻¹ at 298K. This corresponds to a characteristic diffusion length of about 10 μm for 1 minute of discharge.

The present invention also provides working electrodes comprising niobium molybdenum oxide. The working electrode is electrically conductive and can be electrically connected to a counter electrode, eg, in an electrochemical cell.

The working electrode may be the anode or the cathode during the discharge step, for example in a lithium ion battery. Typically the working electrode is the anode during the discharge step.

Typically, the working electrode used in the method contains a molar ratio of _Nb2O5 and _MoO3 from 6 _: 1 to 1:3. Preferably, the molar ratio _{of Nb2O5} and _MoO3 in the working electrode is 1:3.

Typically, _{the working electrode} used in _the method comprises a niobium _molybdenum oxide selected from _Nb2Mo3O14 , _Nb14Mo3O44 , or _Nb12MoO44 . Preferably, _the working electrode _{comprises Nb2Mo3O14} .

Typically, the working electrode does not have a porous or hierarchical structure. The working electrode may have a specific surface area, pore volume and average pore size as described above.

Typically, the working electrode comprises niobium molybdenum oxide in particulate form. The niobium molybdenum oxide particles of the working electrode may have a primary or agglomerate particle size as described above.

The working electrode may comprise binders and adhesives as described above.

Electrochemical Cells The present invention also provides electrochemical cells comprising the working electrode of the present invention. The working electrode may be the anode or the cathode during the discharge step, for example in a lithium ion battery. Typically the working electrode is the anode during the discharge step.

An electrochemical cell typically includes a counter electrode and an electrolyte. An electrochemical cell may include a current collecting plate. The electrochemical cell may be electrically connected to a power source. The electrochemical cell may be electrically connected to a measuring device, such as an ammeter or voltmeter.

The counter electrode may be the anode or the cathode during the discharge step, for example in a lithium ion battery. The counter electrode is typically the cathode during the discharge step.

Suitable cathode materials include lithium-containing or lithium-intercalating materials, such as lithium metal oxides, where the metal is typically a transition metal such as Co, Fe, Ni, V, or Mn, or combinations thereof. include. Some examples of positive _electrode materials are lithium cobalt oxide ( _LiCoO2 ), lithium nickel manganese cobalt oxide ₍ NMC, _LiNiMnCoO2 , e.g. _{LiNi0.6Co0.2Mn0.2O2 )} , fluoride Lithium vanadium phosphate (LiVPO ₄ F), lithium nickel cobalt aluminum oxide (NCA, LiNiCoAlO ₂ ), lithium iron phosphate (LFP, LiFePO ₄ ), and manganese-based spinels (eg LiMn ₂ O ₄ ).

To improve conductivity in the counter electrode, a conductive carbon material (eg, carbon black, graphite, nanoparticulate carbon powder, or carbon nanotubes) is typically mixed with the counter electrode material. In one embodiment, the counter electrode comprises porous carbon, such as porous reduced graphene oxide.

In one embodiment, the counter electrode is substantially binder-free.

In one alternative embodiment, the counter electrode is mixed with a binder or adhesive. Some examples of binders or adhesives include PVDF, PTFE, CMC, PAA, PMMA, PEO, SBR, and copolymers thereof.

The counter electrode is typically fixed to a current collecting substrate, eg an aluminum plate.

Typically, the electrolyte in electrochemical cells is suitable for solubilizing lithium ions.

Typically, electrolytes in charge and discharge cells contain lithium ions.
Typically, the electrolyte is a lithium salt such as LiTFSI, (bis(trifluoromethane)sulfonimide lithium salt, _LiPF6 , _LiBF4 , _LiClO4 , LiTF (lithium triflate), or lithium bis(oxalate)borate (LiBOB). include.

The electrolyte may be a liquid electrolyte, such as one that is liquid at room temperature, eg, 25°C.

The electrolyte may be a non-aqueous electrolyte. The electrolyte may contain a polar aprotic solvent. The electrolyte may contain an organic solvent. Solvents for dissolving lithium ions are well known in the art.

Suitable solvents include carbonate solvents. For example, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), chloroethylene carbonate, fluorocarbonate solvents (e.g. fluoroethylene carbonate and trifluoromethylpropylene carbonate), and dialkyl carbonate solvents such as carbonic acid. dimethyl (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC).

Suitable solvents also include sulfone solvents. For example, methylsulfone, ethylmethylsulfone, methylphenylsulfone, methylisopropylsulfone (MiPS), propylsulfone, butylsulfone, tetramethylenesulfone (sulfolane), phenylvinylsulfone, allylmethylsulfone, methylvinylsulfone, divinylsulfone (vinylsulfone) ), diphenylsulfone (phenylsulfone), dibenzylsulfone (benzylsulfone), vinylenesulfone, butadiene sulfone, 4-methoxyphenylmethylsulfone, 4-chlorophenylmethylsulfone, 2-chlorophenylmethylsulfone, 3,4-dichlorophenylmethylsulfone, 4-(methylsulfonyl)toluene, 2-(methylsulfonyl)ethanol, 4-bromophenylmethylsulfone, 2-bromophenylmethylsulfone, 4-fluorophenylmethylsulfone, 2-fluorophenylmethylsulfone, 4-aminophenylmethylsulfone , sultones (eg, 1,3-propane sultone), and sulfone solvents containing ether groups (eg, 2-methoxyethyl(methyl)sulfone and 2-methoxyethoxyethyl(ethyl)sulfone).

Suitable solvents also include silicon-containing solvents such as siloxanes or silanes. For example, hexamethyldisiloxane (HMDS), 1,3-divinyltetramethyldisiloxane, polysiloxanes, and polysiloxane-polyoxyalkylene derivatives. Some examples of silane solvents include methoxytrimethylsilane, ethoxytrimethylsilane, dimethoxydimethylsilane, methyltrimethoxysilane, and 2-(ethoxy)ethoxytrimethylsilane.

Typically, additives may be included in the electrolyte to improve performance. For example, vinylene carbonate (VC), vinyl ethylene carbonate, allyl ethyl carbonate, t-butylene carbonate, vinyl acetate, divinyl adipate, nitrile acrylate, 2-vinylpyridine, maleic anhydride, methyl cinnamate, ethylene carbonate, halogenated carbonic acid Ethylene, α-bromo-γ-butyrolactone, methyl chloroformate, 1,3-propanesultone, ethylene sulfite (ES), propylene sulfite (PS), vinylethylene sulfite (VES), fluoroethylene sulfite (FES) , 12-crown-4 ether, carbon dioxide (CO ₂ ), sulfur dioxide (SO ₂ ), and sulfur trioxide (SO ₃ ).

The electrochemical cell may also include a solid porous membrane positioned between the negative and positive electrodes. The solid porous membrane may partially or completely replace the liquid electrolyte. Solid porous membranes may be made of polymers (e.g., polyethylene, polypropylene, or copolymers thereof), or inorganic materials, such as transition metal oxides (e.g., titania, zirconia, yttria, hafnia, or niobia), or main group metals. Oxides, such as silicon oxide, which may be in the form of glass fibres, may also be included.

The solid non-porous membrane may comprise a lithium ion conductor. For example, LLZO (garnet family), LSPO (LISICON family), LGPS (thio-LISICON family), LATP/LAGP (NASICON family), LLTO (perovskite family), and phosphide/sulfide glass-ceramics.

The electrochemical cell may be charged or discharged at a C-rate of at least 5 C, for example the electrochemical cell is charged or discharged at a C-rate of at least 5 C for one electron transfer per transition metal per chemical formula unit of the working electrode material. may Preferably, the electrochemical cell may be charged or discharged at a C-rate of at least 10C, 15C, 20C, 25C, 30C, 35C, 40C, 50C, 60C, or 80C.

The electrochemical cell may be charged or discharged at a current density of at least 750 mA·g ⁻¹ . Preferably, the electrochemical cell is at least 800 mA·g ⁻¹ , 850 mA·g ⁻¹ , 900 mA·g ⁻¹ , 950 mA·g ⁻¹ , 1000 mA·g ⁻¹ , 1050 mA·g ⁻¹ , 1100 mA·g ⁻¹ , 1200 mA·g ⁻¹ , or 1300 mA·g ⁻¹ .

The electrochemical cell may have a volume charge density of at least 200, 300, 400, 500, 600, or 700 A·h·L ⁻¹ at 1C. Typically, an electrochemical cell may have a volume charge density of up to 100, 200, 300, or 400 A·h·L ⁻¹ at 20C.

The electrochemical cell is at least It may have a capacity retention of 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or 100%.

An electrochemical cell may be considered fully charged when the voltage exceeds a threshold. For example, an electrochemical cell comprising a lithium metal anode and ^a niobium tungsten oxide cathode may be used if the voltage is raised above practical levels, e.g. If it rises above or above 2.5V, it may be considered fully charged.

An electrochemical cell may be considered fully discharged when the voltage exceeds a threshold. For example, an electrochemical cell comprising a lithium metal anode and a niobium tungsten oxide cathode can be used if the voltage drops below a practical level, e.g., below 1.5 V versus Li/Li ⁺ , e.g., below 1.25 V. Or if it drops below 1.0V, it may be considered fully discharged.

The electrochemical cell may be a lithium ion battery.

Methods The present invention provides a method of charging and/or discharging an electrochemical cell at a C-rate of at least 5C, e.g. It may be charged or discharged at the C rate. Electrochemical cells include working electrodes comprising niobium tungsten oxide and/or niobium molybdenum oxide. Preferably, the electrochemical cell includes a counter electrode and an electrolyte.

Preferably, the method is a method of charging and/or discharging the electrochemical cell at a C-rate of at least 10C, 15C, 20C, 25C, 30C, 35C, 40C, 50C, 60C, or 80C.

The method may be a method of charging and/or discharging the electrochemical cell at a current density of at least 750 mA·g ⁻¹ , such as at least 800 mA·g ⁻¹ . Preferably, the method comprises at least 800 mA·g ⁻¹ , 850 mA·g ⁻¹ , 900 mA·g ⁻¹ , 950 mA·g ⁻¹ , 1000 mA·g ⁻¹ , 1050 mA·g ⁻¹ , 1100 mA·g ⁻¹ , 1200 mA • g ⁻¹ , or a method of charging and/or discharging an electrochemical cell at a current density of 1300 mA·g ⁻¹ .

The method includes charging and discharging, or discharging and charging, the electrochemical cell. The cycle may be repeated more than once. Thus, methods include 2 or more cycles, 5 or more cycles, 10 or more cycles, 50 or more cycles, 100 or more cycles, 500 or more cycles, 1000 or more cycles, or 2000 or more cycles.

Batteries The invention also provides batteries comprising one or more electrochemical cells of the invention. The battery may be a lithium ion battery.

If multiple cells are present, these may be provided in series or parallel.

The battery of the invention may be provided in road vehicles such as automobiles, motorized bicycles or trucks. Alternatively, the battery of the invention may be provided in a rail vehicle, such as a train or tram. A battery of the invention may also be provided in an electric bicycle (e-bike).

A battery of the present invention may be provided in a regenerative braking system.

The battery of the invention may be provided in portable electronic devices such as mobile phones, laptops or tablets.

A battery of the invention may be provided in a power grid management system.

Uses The present invention generally provides the use of working electrodes comprising niobium tungsten oxide in high speed electrochemical cells, such as electrochemical cells as described herein. Typically, the electrochemical cell may be charged or discharged at a C-rate of at least 5C, for example the electrochemical cell has a C-rate of at least 5C for one electron transfer per transition metal per chemical formula unit of the working electrode material. may be charged or discharged with Preferably, the electrochemical cell may be charged or discharged at a C-rate of at least 10C, 15C, 20C, 25C, 30C, 35C, 40C, 50C, 60C, or 80C.

The electrochemical cell may be charged or discharged at a current density of at least 750 mA·g ⁻¹ . Preferably, the electrochemical cell is at least 800 mA·g ⁻¹ , 850 mA·g ⁻¹ , 900 mA·g ⁻¹ , 950 mA·g ⁻¹ , 1000 mA·g ⁻¹ , 1050 mA·g ⁻¹ , 1100 mA·g ⁻¹ , 1200 mA·g ⁻¹ , or 1300 mA·g ⁻¹ .

A working electrode may be used in the methods described herein.

Other Preferences Any compatible combination of the above embodiments is expressly disclosed herein, as if every combination were individually and explicitly recited.

Various further aspects and embodiments of the invention will be apparent to those skilled in the art in view of the present disclosure.

"and/or" when used herein is considered specific disclosure of each of the two specified features or components, whether or not the other is present. For example, "A and/or B" refers to the specific disclosure of each of (i) A, (ii) B, and (iii) A and B as if each of them were individually listed herein. is considered.

Unless the context dictates otherwise, the descriptions and definitions of the features set forth above are not limited to any particular aspect or embodiment of the invention, and all aspects and embodiments described. apply equally.

Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the above drawings.

EXPERIMENTAL The following examples are provided only to illustrate the invention and are not intended to limit the scope of the invention described herein.

Synthesis of Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ were combined with dark blue NbO ₂ (Alfa Aesar, 99+%) and brown WO ₂ (Alfa Aesar, 99.9%) by co-thermal oxidation in about 1-5 gram batches. The partially reduced oxides were agglomerated to within 0.001 g of a 16:5 or 18:16 molar ratio, manually ground together using an agate mortar and pestle, pressed into pellets at 10 MPa, and platinum It was heated in a crucible to 1473 K at a rate of 10 K·min ⁻¹ and allowed to cool naturally in a furnace over about 2 hours.

Synthesis of Nb ₈ W ₉ O ₄₇ and Nb ₁₂ WO ₃₃ Nb ₈ W ₉ O ₄₇ and Nb ₁₂ WO ₃₃ were combined with dark blue NbO ₂ (Alfa Aesar, 99+%) and brown WO ₂ (Alfa Aesar, 99 .9%) by cothermal oxidation in about 1 to 5 gram batches. The partially reduced oxides were agglomerated to within 0.001 g of a molar ratio of 8:9 or 12:1, manually ground together using an agate mortar and pestle, pressed into pellets at 10 MPa, and platinum It was heated in a crucible to 1473 K at a rate of 10 K·min ⁻¹ and allowed to cool naturally in a furnace over about 2 hours.

Synthesis of Nb ₂ Mo ₃ O ₁₄ Nb ₂ Mo ₃ O ₁₄ was prepared in about 1 to 5 gram batches of dark blue NbO ₂ (Alfa Aesar, 99+%) and dark brown MoO ₃ (Sigma, 99%). ) or co-thermal oxidation of white Nb ₂ O ₅ (Sigma, 99.9985%) and off-white MoO ₃ (Sigma, ≧99.5%). The partially reduced oxides were agglomerated to within 0.001 g of a 2:3 or 1:3 molar ratio, manually ground together using an agate mortar and pestle, pressed into pellets at 10 MPa, and platinum or heated to 873 K, 923 K, or 973 K at a rate of 10 K·min ⁻¹ in an alumina crucible and quenched on a metal plate in air outside the furnace.

Microscopic Characterization Scanning electron microscopy (SEM) images were taken with a Σigma VP instrument (Zeiss) at 3.0 kV and a MIRA3 instrument (TESCAN) at 5.0 kV with secondary electron detection. Tap densities were recorded on an AutoTap (Quantachrome Instruments Inc.) instrument operating at 257 taps min ⁻¹ . Tap density was measured according to ASTM international standard B527-15 modified to accommodate a 5 to 10 cm ³ graduated cylinder.

Both Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ have grains of about 3 to 10 μm that appear to intergrow/“bond” into larger secondary particles of about 10 to 30 μm under electron microscopy. Sub-hedral particles were formed by primary particles (FIGS. 1 and 2). The bulk compound is a series of complex _{'block' (Nb 16 W 5 O 55 )} or 'bronze-like _' (Nb ₁₈ W ₁₆ O ₉₃ ) oxide structure (FIG. 2).

Nb ₁₆ W ₅ O ₅₅ is a unit composed of corner-sharing octahedral subunits arranged into ReO ₃ -like blocks 4 octahedra wide by 5 octahedra long and infinite in the third dimension. It has an orthorhombic structure (Fig. 2A). The block subunits are connected by crystallographic shear planes along the edges and by tetrahedra at each corner, hence the notation (4×5) ₁ , where the notation (m× n) In _p , m and n represent the block length in octahedral units, p relates to the connectivity of the block, which may be connected in pairs (p=2) or infinite ribbons (p=∞).

Nb ₁₈ W ₁₆ O ₉₃ is an orthorhombic, 1×3×1 superstructure of classical tetragonal tungsten bronze (TTB) (FIGS. 2D and 3). TTB structures _are sometimes stabilized by cations (e.g., _K +, partially filled spheres in Fig. ₃ ); , stabilized by -MO- chains that form a pentagonal bipyramid. The effect of ions and electrons on lithium intercalation, respectively, is that the layers of Nb ₁₈ W ₁₆ O ₉₃ are still fully open and unoccupied by cations (i.e., around the plane (x, y, 1/2) and that, although all metal cations are in the desired d ⁰ electron configuration, neither of these criteria are met in the classical TTB structure.

Electrochemical characterization of Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ (Cambridge Energy Solutions, Inc.) with stainless steel 2032-coin cells (Cambridge Energy Solutions, Inc.). To form a niobium tungsten oxide electrode, niobium tungsten oxide and conductive carbon (Super P, TIMCAL) were manually ground with an agate mortar and pestle at a weight ratio of 8:1. This powder was ground in a 9:1 weight ratio with poly(vinylidene difluoride) (PVDF, Kynar) dispersed in N-methylpyrrolidone (NMP, Sigma-Aldrich, anhydrous, 99.5%). The slurry was coated onto aluminum or copper foil using a doctor blade (bar coater). NMP was removed by heating in an oven at 60° C. for 24 hours. As standard, Super P carbon is a nanoparticulate powder and NMP is a hazardous organic solvent, so a suitable nanoparticle cabinet/fume hood should be used.

This 80/10/10 metal oxide/carbon/polymer electrode served as the cathode to a Li metal disk (LTS Research, 99.95%) anode in a half-cell configuration. In the electrochemical tests, the electrolyte was 1M LiPF ₆ dissolved in 1:1 v/v ethylene carbonate/dimethyl carbonate (EC/DMC; Sigma-Aldrich, battery grade; also known as LP30). No additives were used. Electrochemical measurements were performed in a temperature-controlled room at 293K. Electrochemical measurements were performed using a Biologic galvanostat/potentiostat with EC-Lab software.

Large-grain dense electrodes with active mass loadings of 2 to 3 mg·cm ⁻² were tested at current densities corresponding to discharge times of hours to tens of seconds. Nb ₁₆ W ₅ O ₅₅ was charged with a constant voltage step of 1 h at the top of the charge to ensure a comparable starting point of discharge; Nb ₁₈ W ₁₆ O ₉₃ was cycled without this step and Stored at >100 mA·h·g ⁻¹ (ie, <60 s) at 60C. 1000 cycles of fast cycling were performed on both oxides at 10C/20C constant current without any potentiostatic step.

The reaction _{of Nb16W5O55} with _lithium (Fig _{. 4A} ) _shows a 3 It progresses in two regions (Fig. 5). Three regions are more readily observed in the derivative plot (Fig. 4B), characterized by their slopes and other crystallographic shear structures (e.g., H—Nb ₂ O ₅ , PNb ₉ O ₂₅ , TiNb ₂ O ₇ , and Nb ₁₂ WO ₃₃ ). Nb ₁₆ W ₅ O ₅₅ exhibited unprecedented bulk velocity performance when kinetics were investigated over the current density range from C/5 (34.3 mA·g ⁻¹ ) to 60 C (10.3 A·g ⁻¹ ) was shown (FIGS. 4A and 4C). At C/5, about 1.3 lithium ions can be reversibly intercalated per transition metal for a gravimetric capacity of about 225 mA·h·g ⁻¹ . When the rate is increased 25 times to 5C, the Nb ₁₆ W ₅ O ₅₅ maintains a capacity of 1.0 Li ⁺ /TM (171 mA·h·g ⁻¹ ). At 20 C, corresponding to 3 minutes of discharge, it is still possible to exchange 0.86 Li ⁺ /TM and reach 148 mA·h·g ⁻¹ . Rate tests of Nb ₁₆ W ₅ O ₅₅ were measured with a constant potential hold at the top of the charge to ensure a reliable starting point of discharge. To test performance under more severe conditions, 1000 cycles were measured with fixed constant current discharge and charge conditions of 250 cycles at 10 C followed by 750 cycles at 20 C without holding (Fig. 4F). Under these conditions, 0.90 Li ⁺ /TM (average 155 mA·h·g ⁻¹ ) at 10 C with 95% capacity retention after 250 cycles on non-optimized or calendered electrodes. reversibly intercalated with At 20C, the capacity was 0.75 Li ⁺ /TM (average 128 mA·h·g ⁻¹ ); capacity retention was again 95% over 750 cycles at 20C.

The average voltage of Nb ₁₈ W ₁₆ O ₉₃ is 1.67 V (FIGS. 4D-4F). In terms of gravimetric capacity, Nb ₁₈ W ₁₆ O ₉₃ stores about 20 mA·h·g ⁻¹ less than Nb ₁₆ W ₅ O ₅₅ at C/5 and 1C due to the higher molar mass of the tungsten-rich bronze phase. . However, at 20C, Nb ₁₈ W ₁₆ O ₉₃ can still accommodate total unit Li+/TM for a capacity of about 150 mA·h·g ⁻¹ . At 60 C and 100 C (14.9 A·g ⁻¹ ) the capacities are still 105 and 70 mA·h·g ⁻¹ respectively.

Comparison of Cu Foil Current Collectors and Carbon Coated Al Foil Current Collectors FIG. 6 shows the impact of the current collectors at high speeds which cannot be ignored.

For 1000 cycles of Nb ₁₆ W ₅ O ₅₅ under galvanostatic discharge and charge at 10 C/20 C, the Cu foil current collector showed moderately higher capacity than the carbon-coated Al (C@Al) foil. (Fig. 6A). Untreated Al foil has been shown to be inadequate for high current densities (Griffith et al.); C@Al showed considerable improvement over untreated Al. In Nb ₁₆ W ₅ O ₅₅ , the lower voltage region is the source of excess capacity above 1.0 Li+/TM at low speeds, but effectively shifts to below 1.0 V at moderate speeds. A second discharge peak (centered at 1.2 V) is only observed at C/5 in the dQ/dV plot (Fig. 4B). As a result, the capacities observed for cycling at lower voltages of 1.2 V vs. 1.0 V become less significant as speed increases, and avoidance of low voltages may further improve safety. have a nature. FIG. 6B shows the discharge and charge profiles between 3.0 and 1.0 V for the 100th cycle at 10C.

Longer-Term Cycling as a Function of Minimum Cut-Off Voltage FIG. 7 shows the effect of long-term cycling at C/5.

Constant-current constant-voltage charging, as in (Fig. 7A), is a popular method to maximize capacity without overcharging, but the extra time required means that cycling can and 1C may be considered discharging, but C/6 and C/2 may be considered charging. The retention rates for the 2nd to 50th cycles in FIG. 7A are 96% and 93% for the lowest voltages of 1.2 and 1.0 V, respectively. The 51st to 100th cycles at 1C show capacity retention of 99% and 101% for 1.2 and 1.0 V, respectively. The cycle retention ratios for the 2nd to 50th cycles for C/5 in FIG. 7B are 88%, 90% and 40% for 1.1, 1.0 and 0.9V respectively. Cycling time was approximately 1 hour.

Overpotentials in Li||Li Symmetric Cells as a Function of Current Density As a control, Li||Li symmetric cells were cycled at current densities corresponding to those in FIGS. 4A-4E (FIG. 8). The cell was arranged identically to that used for the metal oxide tests, except that the composite electrode was replaced with a second Li disk. Rate tests were performed at 100 μA (C/5), 500 μA (1 C), 1 mA (2 C), 2.5 mA (5 C), 5 mA (10 C) for 5 cycles and 10 mA (20 C), 20 mA (40 C), 30 mA (60 C). ), and 10 cycles at 50 mA (100C). "Speed" in brackets indicates the reciprocal of the number of hours the current was applied, simulating a speed test. At low current densities below 1 mA (2C) the overpotential is less than 100 mV; however, at 5 mA (10C) the overpotential rises to 200 mV and at 100C increases to about 700 mV.

The overpotentials in the symmetrical cell (Fig. 8) are in good agreement with those observed in the electrochemical cycling curves of Figs. 4A and 4D. This is because the extremely high rates for bulk electrodes approach the limits of Li metal plating/stripping and/or desolvation and transport of lithium ions in carbonate electrolytes at room temperature, i.e. the ohmic It suggests that a substantial portion of the loss is due to the Li metal rather than the composite oxide electrode material.

⁷ Li pulsed field gradient NMR spectroscopy
⁷ Li NMR diffusion spectra were recorded on a Bruker Avance III 300 MHz spectrometer using a Diff50 probehead with extended variable temperature capability. Spectra were recorded with the stimulated-echo pulse magnetic field gradient (PFG) sequence shown in FIG. 9 to minimize spin-spin relaxation time (T ₂ ) loss. After the first 90° radio frequency (rf) pulse, the net magnetization loses coherence due to _T2 relaxation; therefore, the period following this pulse, including the first PFG pulse (to encode the spin position), is _T2 must be shorter. In the stimulated-echo sequence used here, we applied a second 90° rf pulse that stored the net magnetization along the z-axis prior to the diffusion time, Δ, and no _T2 relaxation occurred, so the observed It allows the species to diffuse for a time commensurate with the relatively long spin-lattice (T ₁ ) value. During Δ, apply a short spoiler gradient (SINE.100) to eliminate residual transverse magnetization. A third 90° rf pulse is then applied, followed by a PFG pulse to decode the spin positions. A sufficiently long delay (≧0.5 ms) between the PFG and the radiofrequency pulse was used to minimize eddy currents in the diffusion measurements.

During this sequence, the gradient strength, g, was varied from 0.87 to 1800 G cm ⁻¹ or 2300 G cm ⁻¹ , using “opt” shaped pulses with transients from 1024 to 4096 for 16 Got a gradient step. The optoshape is a composite pulse that begins with a quarter sine wave followed by a constant slope and ends with a ramp down (Fig. 9). An 'opt' gradient pulse provides the maximum gradient integral for a given time period, maximizing the range of diffusion coefficients that could be evaluated in this experiment.

The spectra were analyzed in phase-sensitive mode and the response of the NMR signal intensity, I, is expressed by the Stejskal-Tanner equation for variations in g:

where I ₀ is the intensity without gradient, γ is the gyromagnetic ratio (γ _7Li =103.962×10 ⁶ s ⁻¹ ·T ⁻¹ ), and δ is the effective gradient pulse width. , D is the diffusion coefficient. Here, NMR signal intensity and integration gave similar ⁷ Li diffusivities, but NMR signal intensity gave more reliable data as assessed by the standard deviation of the fit. Typical δ values ranged from 0.8 ms to 1.5 ms and Δ values ranged from 50 to 100 ms for bronze and block phase samples, respectively.

Diffusion spectra were recorded at high temperature (333-453 K) due to the observed _T2 increase at high temperature (e.g., the _T2 of Li _3.4 Nb ₁₈ W ₁₆ O ₉₃ is about 700 μs at room temperature, 1.9 ms at 453K). (Note: No attempt was made to calibrate the temperature for this experimental set-up because a monotonic ⁷ Li coil was used and a reliable ⁷ Li reference is not routinely used for temperature calibration. The Bruker manual states: It is stated that for static measurements the temperature calibration should be within ±7 degrees of the set point.) An increase in _T2 was necessary to measure diffusion coefficients in solid oxides. allowed the use of longer gradient pulses, δ.

A representative ⁷ Li diffusion decay curve is shown in FIG. Data quality directly correlates to the Li content and the signal-to-noise ratio (SNR) of subsequent ⁷ Li NMR spectra, where SNRs of 95, 56, 10, and 12, respectively, are shown in Figure 10A (Li _10.2 Nb _{18 W16O93 ) ,} FIG _. 10B _{( Li6.8Nb18W16O93} ) _{, FIG .} 10C ( _{Li3.4Nb18W16O93} ), and _{FIG . 10D} ( _{Li6.3Nb16W5O55} ) was observed. The SNR of the low Li content Li _3.4 Nb ₁₈ W ₁₆ O ₉₃ sample was low, with a signal attenuation of only about 60%, whereas the observed trends in ⁷ Li diffusivity and activation energy were We note that it is consistent with what was observed in the more lithiated bronze structure.

Li _6.3 Nb ₁₆ W ₅ O ₅₅ exhibits binary behavior with a rapid lithium transport of 4.3×10 ⁻¹² m ² ·s ⁻¹ at 333 K (FIG. 11). Assuming Arrhenius behavior and a measured activation energy of 0.23 eV, the lithium diffusion coefficient at room temperature is estimated to be 2.1×10 ⁻¹² m ² ·s ⁻¹ (Table 1). .

Li _x Nb ₁₈ W ₁₆ O ₉₃ (x=3.4, 6.8, 10.2) has diffusion coefficients of 1.1×10 ⁻¹³ m ² ·s ⁻¹ and 0.27 to 0.27 at room temperature. They exhibited similar diffusion and activation energies, with E _a in the range of 30 eV.

Lithium diffusion in both niobium tungsten oxide structures is significantly faster than that of Li _4+x Ti ₅ O ₁₂ or Li _x TiO ₂ , about 10 ⁻¹⁶ to 10 ⁻¹⁵ m ² ·s ⁻¹ , and is best known. It is close to a lithium solid electrolyte (Table 2).

PFG NMR (Fig. 11) results indicate that the initial lithium ion diffusion is on the order of ^10-12 to ^{10-13 m2s -1} , while galvanostatic intermittent titration (GITT) measurements (Fig. 12) show that niobium tungsten oxide sustains this rapid kinetics up to high capacities (≧1.5 Li ⁺ /TM), where diffusion drops off by about two orders of magnitude towards 2.0 Li ⁺ /TM. . This suggests that the inherent range of niobium tungsten oxide electrode materials for fast multi-redox extends to about 1.5 Li ⁺ /TM. Diffusion coefficients on the order of 10 ⁻¹² to 10 ⁻¹³ m ² ·s ⁻¹ measured for these materials are consistent with the values required to achieve complete lithiation of 6 to 19 μm particles on the 60 C timescale. .

Galvanostatic intermittent titration (GITT)
Information on electrode thermodynamics and lithium kinetics, including phase transitions, can be obtained in principle by following the voltage evolution after a short current pulse as lithium diffuses and the chemical potential equilibrates within the electrode/particle. It can be extracted from GITT measurements. However, it is difficult to extract a reliable quantitative diffusion coefficient, D _Li , from GITT alone. In order to determine the diffusion coefficient from GITT measurements, the diffusion length (L) must be determined, and battery electrodes are heterogeneous systems. First, the battery electrode is a composite of active material (here, a metal oxide), porous carbon, and a polymer binder. Within this composite there is a distribution of grain sizes (unless single crystals or well-defined grains are used; even then diffusion varies with lattice direction). Furthermore, since there are solid/liquid interfaces and porous electrode structures, different routines of diffusion must exist. Nevertheless, in electrodes that do not undergo severe crushing (eg, intercalation electrodes), L is a fixed quantity throughout the experiment. Variations in L, a parameter necessary to relate the rate of relaxation to diffusion, cause the values of _DLi to vary considerably between reports, even for the same material. Therefore, although a physically meaningful diffusion coefficient may not be extracted, a relative measure of diffusion is readily obtained. For this reason, an extracted proxy for lithium diffusion (D _Li L ^-2 , Fig. 12). The addition of quantitative information from other methods, such as NMR spectroscopy, allows relative changes in Li ⁺ kinetics to be calibrated against quantitative diffusion values over a range of lithiations. .

As shown in FIG. 12, the fastest diffusion is observed in Nb ₁₆ W ₅ O ₅₅ from the dilution limit of Li _4.5(5) Nb ₁₆ W ₅ O ₅₅ , incorporating Li ⁺ /TM of 2 or more. There was a two order of magnitude drop in the lower voltage window. The GITT data indicate that the second electrochemical region of Nb ₁₆ W ₅ O ₅₅ is broader than that typically observed for two-phase reactions, although the observed D _Li L ⁻ A binary discontinuity suggests approaching ^two -phase behavior. The average diffusion coefficient of Nb ₁₈ W ₁₆ O ₉₃ is similar to that of Nb ₁₆ W ₅ O ₅₅ . Bronze also shows discontinuities at 2.1, 1.85 and 1.7V. In both phases, the low voltage region, below 1.25 V and well above 1 Li ⁺ /TM, is characterized by increasing overpotentials and suppressed kinetics.

Anode Material Ragone Plot Anode Material Ragone Plot: The energy density of a cathode material is the product of capacity (Q) and voltage (V); however, this product compares anode materials where energy and voltage are inversely related. doesn't work in that case. In calculating the anode material Ragone plot of FIG. 13, the energy (E) is calculated based on the voltage difference to the 4.0V cathode. Therefore, normalized to the anode, E _anode = (V _cathode - V _anode )*Q _anode .

Titania, niobia, and graphite all exhibit theoretical charge densities greater than 800 A·h·L ⁻¹ ( Figure 14). Considering experimental volumes and tap densities (Fig. _13A ), the bulk, non-optimized niobium tungsten oxides presented here, even optimized versions of _TiO2 , _Nb2O5 , LTO It maintains volumetric charge densities greater than 500 A·h·L ⁻¹ at 1C and up to 400 A·h·L ⁻¹ at 20C, unattainable volumetric performance. This is not to say that the proposed compounds cannot be improved by methods such as nanostructuring, calendering, or carbon coating, as evidenced by porous graphene scaffolds, for example by Sun et al. , demonstrate that large micrometer-sized particles can be used in high-speed electrodes, and demonstrate that nanosizing is not necessarily the most appropriate strategy to improve performance. This is evident in the Ragone plot (Fig. 13B), which shows the higher energy density of the new bulk niobium tungsten oxide when compared to state-of-the-art fast anode materials and graphite.

Commercial Materials—Half Cell Test To further test the suitability of niobium tungsten oxide as a high speed anode material, a commercial high speed cathode material NMC622 (LiNi _0.6 Mn _0.2 Co _0.2 O ₂ ; Targray) was used. , lithium iron phosphate ( _LiFePO4 ; Johnson _Matthey ), and _LiMn2O4 (MTI Corp) were purchased. These commercial materials were first characterized against Li metal in half-cell geometry. Electrochemical measurements were performed using stainless steel 2032-coin cells and glass microfiber separators in the same manner as for Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ described above. Commercially available cathode materials, conductive carbon (Li-250 carbon; Deka Chemicals) and PVDF were ground together in the same manner as above to form an 80/10/10 electrode ( 80 wt% active material, 10 wt% carbon and 10 wt% polymer) was prepared. The electrolyte was 150 μL LP30. No additives were used.

NMC622 exhibited an average voltage of 3.8 V and a practical capacity of 175 mA·h·g ⁻¹ under these conditions (FIG. 17A). LiFePO ₄ exhibited an average voltage of 3.4 V and a practical capacity of 165 mA·h·g ⁻¹ (FIG. 17B), and LiMn ₂ O ₄ exhibited an average voltage of 4.0 V and a practical capacity of 120 mA·h·g ⁻¹ . capacity was shown (Fig. 17C).

Electrode optimization—Nb ₁₆ W ₅ O ₅₅
To optimize the performance of the Nb ₁₆ W ₅ O ₅₅ electrodes, a series of electrodes were fabricated with the specifications given in Table 3 below. Electrochemical characterization was performed using stainless steel 2032 coin cells and glass microfiber separators as described above. A Li metal disk was used as the anode in a half-cell configuration. The electrolyte was 150 μL LP30. No additives were used.

Bulk velocity performance was shown to improve when 5 wt% PVDF or 4 wt% SRB and 1 wt% CMC were used as binders (Figure 18). Calendering also consistently improved electrode performance.

Full Cell Operation with Commercial Electrode Materials To test the suitability of niobium _tungsten oxide as a high speed anode material, a full cell was fabricated using the commercial high speed cathode materials NMC622, _LiFePO4 , and _LiMn2O4 . Electrochemical measurements were performed using stainless steel 2032-coin cells and glass microfiber separators in the same manner as for Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ described above. Commercially available cathode materials, Li-250 carbon and PVDF, were ground together as described above to form an 80/10/10 electrode (80 wt% active material, 10 wt% carbon, and 10 wt% polymer including) were prepared. Nb ₁₆ W ₅ O ₅₅ , Li-250 carbon and PVDF were ground together to form an 80/10/10 electrode (80 wt% active material, 10 wt% carbon, and 10 wt% polymer) that served as the anode. including) were prepared. Where the cathode contained NMC622, the anode contained Nb ₁₆ W ₅ O ₅₅ , Li-250 carbon and PVDF in a ratio of 80/15/5. The electrolyte was 150 μL LP30 in all cases. No additives were used. The capacity ratio of the anode and cathode was 1:1. For full-cell balancing purposes, the theoretical capacity of NMC622 was assumed to be 175 mA·h·g ⁻¹ .

Full cells were rated between 1.0 and 3.0V. The initial charge/ _discharge curves of full cells with _LiFePO4 and _LiMn2O4 cathodes are shown in Figures 19A and 19B, respectively. At rates as low as C/5, the full cell exhibited a capacity of 120 mA·h·g ⁻¹ for LiMn ₂ O ₄ , which is close to the practical capacity of the cathode. A capacity of 140 mA·h·g ⁻¹ was measured with LiFePO ₄ as the cathode. A high capacity of 165 mA·h·g ⁻¹ was measured with NMC622 as the cathode (FIG. 19C). Capacity is estimated based on the mass of the active cathode material.

The speed performance of the NMC622 full cell was evaluated in the current density range from C/5 to 20C. The NMC622 full cell maintained a capacity of 125 mA·h·g ⁻¹ at 20 C, greater than 75% capacity retention relative to capacity at C/5 (FIG. 19D). To test longer term cycling, NMC622 full cells were cycled at 10C charge and 10C discharge rates for 300 cycles. Under these conditions, a capacity of 120 mA·h·g ⁻¹ was observed after 300 cycles, corresponding to over 80% capacity retention (FIG. 19E). At lower rates, a very high capacity retention of over 98% was achieved for 100 cycles at 1C charge and 1C discharge (Fig. 19F).

Investigation of Electrode Degradation To investigate the cause of capacity loss, full cells with Nb ₁₆ W ₅ O ₅₅ anodes and NMC622 cathodes as described above were subjected to 500 cycles of 1C charge and 1C discharge. A capacity retention of 71.1% was observed (Figure 20A). The cell was disassembled and the cathode and anode reassembled into a cell against the Li metal electrode in a half-cell configuration. As shown in FIG. 20B, the extracted NMC622 electrode exhibited a capacity of 0.48 mAh. The initial capacity of the NMC622 half-cell in this configuration was 0.72 mAh. Thus, the NMC622 electrode exhibited 67% capacity retention (33% capacity loss). In contrast, the extracted Nb ₁₆ W ₅ O ₅₅ electrode exhibited a capacity of 0.62 mAh for an initial capacity of 0.73 mAh in this configuration (FIG. 20C). This corresponds to 85% capacity retention (15% capacity loss). Thus, more than half of the capacity loss observed in full cells containing Nb ₁₆ W ₅ O ₅₅ anodes and NMC622 cathodes resulted from degradation of the NMC622 cathodes. This indicates that the Nb ₁₆ W ₅ O ₅₅ material is a particularly resilient electrode material.

Mixtures of Nb ₁₆ W ₅ O ₅₅ and LTO as Anodes Additional experiments were performed to test the use of mixtures of Nb ₁₆ W ₅ O ₅₅ and LTO as electrode materials. Electrodes containing three different ratios of Nb ₁₆ W ₅ O ₅₅ to LTO were prepared and applied to Li metal anodes in the same manner as for Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ described above. measured against. Galvanostatic charge-discharge curves are shown in FIG. 21A normalized to gravimetric capacity. The average voltage of 1.55 V is close to that of pure LTO (Fig. 5A). At a ratio of 7:3 (Nb ₁₆ W ₅ O ₅₅ :LTO), a gravimetric capacity of 225 mA·h·g ⁻¹ was observed at C/5 and 125 mA·h·g ⁻¹ at 20C (Fig. 21B). For 3:7 (Nb ₁₆ W ₅ O ₅₅ :LTO), a gravimetric capacity of 190 mA·h·g ⁻¹ was observed at C/5 and 130 mA·h·g ⁻¹ at 20C (FIG. 21B). . To test longer term cycling, the cells were cycled at 10C charge and 10C discharge rates for 500 cycles. Under these conditions, a capacity of 150 mA·h·g ⁻¹ was observed, indicating greater than 93% capacity retention (FIG. 21C).

_{Glass microfiber _ _ _ _ _ _ _} Prepared in a stainless steel 2032-coin cell with a separator. Nb ₁₆ W ₅ O ₅₅ , LTO, Super P carbon, and PVDF were ground together to prepare an 80/15/15 active material/carbon/polymer anode. NMC622, Li-250 carbon, and PVDF were ground together as above to prepare an 80/10/10 active material/carbon/polymer electrode to serve as the cathode. The electrolyte was 150 μL LP30. No additives were used.

A full cell was rated between 1.0 and 3.25 V as shown in FIG. 22A. At a rate as low as C/5, a capacity of 200 mA·h·g ⁻¹ was observed. The average voltage was 2.25V. Full-cell speed performance was evaluated in the current density range from C/5 to 20C. The full cell maintained a capacity of 160 mA·h·g ⁻¹ at 20 C, greater than 78% capacity retention for capacity at C/5 (FIG. 22B).

Electrochemical Characterization of Nb ₂ Mo ₃ O ₁₄ Electrochemical characterization of Nb ₂ Mo ₃ O ₁₄ was performed using a conical spring, two 0.5 mm stainless steel spacer discs, a plastic gasket, and a glass microfiber separator (Whatman (Cambridge Energy Solutions) in the same manner as for Nb ₁₆ W ₅ O ₅₅ and Nb ₁₈ W ₁₆ O ₉₃ described above. Metal oxides and conductive carbon (Super P, TIMCAL) were manually ground in an agate mortar and pestle at a weight ratio of 8:1. This powder was ground in a 9:1 weight ratio with poly(vinylidene difluoride) (PVDF, Kynar) dispersed in N-methylpyrrolidone (NMP, Sigma-Aldrich, anhydrous, 99.5%). This metal oxide/carbon/polymer electrode served as a cathode to a Li metal disk (LTS Research, 99.95%) anode in a half-cell configuration. The electrolyte was 1M LiPF ₆ dissolved in 1:1 v/v ethylene carbonate/dimethyl carbonate (EC/DMC, Sigma-Aldrich, battery grade). No additives were used. Electrochemical measurements were performed in a temperature-controlled room at 293K. Electrochemical measurements were performed using a Biologic galvanostat/potentiostat with EC-Lab software.

At C/20, it is possible to maintain a gravimetric capacity of about 200 mA·h·g ⁻¹ (FIG. 15). Nb ₂ Mo ₃ O ₁₄ shows an average voltage of about 2.0V. Thus, Nb ₂ Mo ₃ O ₁₄ may be used, for example, in the voltage window of 3.0 to 1.4 V, or 3.0 to 1.8 V, while still providing high capacity at high speeds, while Avoid or minimize interactions with electrolytes. This is an advantage when many prospective solid electrode materials work with low voltage anodes, which can be mitigated using Nb ₂ Mo ₃ O ₁₄ anodes.

REFERENCES All documents mentioned herein are hereby incorporated by reference in their entirety.

Claims (16)

A method of charging and/or discharging an electrochemical cell, comprising:
The method, wherein the electrochemical cell has _a working _{electrode comprising} a niobium tungsten oxide selected from _Nb16W5O55 , _Nb18W16O93 , and combinations _thereof . 2. The method of claim 1, wherein the electrochemical cell is charged and/or discharged at a C-rate of at least 5<0> C. 3. A method according to claim 1 or 2, wherein the electrochemical cell is charged and/or discharged at a current density of at least 750 mA.g ^{- 1} . 4. The method according to any one of claims 1 to ₃ , wherein the ratio of _Nb2O5 and _WO3 in the working electrode is from 8: 5 to 11:20. _{The working electrode comprises Nb12WO33 , Nb26W4O77 , Nb14W3O44 , Nb18W8O69 , Nb2WO8 , Nb22W20O115 , Nb8W9O47 , _ 4.} The method of claims 1-3 _further comprising a _{niobium tungsten} oxide _selected from _Nb54W82O381 , _Nb20W31O143 , _{Nb4W7O31 , Nb2W15O50} , and _combinations thereof _. A method according to any one of paragraphs . 6. The method of any one of claims 1-5 , wherein the working electrode comprises a mixture of niobium tungsten oxide and an additional active material. charging and discharging or discharging and charging the electrochemical cell;
7. The method of any one of claims 1-6 , comprising two or more cycles. An electrochemical cell comprising a working electrode comprising _{a niobium tungsten oxide selected from Nb16W5O55 , Nb18W16O93 ,} and _combinations thereof _{. 9.} An electrochemical cell according to claim 8 , wherein the ratio of _Nb2O5 and _WO3 in said electrode is from 8:5 to 11:20. _{The working electrode comprises Nb12WO33 , Nb26W4O77 , Nb14W3O44 , Nb18W8O69 , Nb2WO8 , Nb22W20O115 , Nb8W9O47 , _ 10.} The method of claim 9 , further comprising _{a niobium} tungsten oxide selected from _Nb54W82O381 , _{Nb20W31O143 , Nb4W7O31 , Nb2W15O50} , and _{combinations thereof .} electrochemical cell. 11. An electrochemical cell according to any one of claims 8-10 , wherein the working electrode comprises a mixture of niobium tungsten oxide and an additional active material. 12. An electrochemical cell according to any one of claims 8 to 11 , wherein the electrodes have neither a porous structure nor a hierarchical structure. 13. An electrochemical cell according to any one of claims 8 to 12 , wherein the electrode comprises niobium tungsten oxide in particulate form. A lithium ion battery comprising one or more electrochemical cells according to any one of claims 8 to 13 . A vehicle comprising the lithium ion battery of claim 14 . Use of a working _electrode comprising _{a niobium tungsten oxide selected from Nb16W5O55 ,} Nb18W16O93 _, and combinations thereof in _a fast _{electrochemical} cell .

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